Digital video is increasingly becoming ubiquitous with wide ranging applications in entertainment, education, business and broadcasting. As such, digital media such as the digital versatile disc (DVD) for example, have been rapidly replacing older analog technologies such as the video home system (VHS), as the preferred means for the distribution of audiovisual content.
The DVD became a popular medium of digital content distribution since it allowed convenient distribution of large amounts of recorded digital data including video, audio, software and electronic documents at a relatively low cost. This is due in part, to the fact that digital processing of video signals has benefited significantly from advances in integrated circuits. The decreasing cost and increasing power of integrated circuits have readily translated into quality improvements in digital video players at reduced costs.
Nonetheless, movies distributed using DVDs are of standard definition (SD) resolution and lately, higher resolution formats such as Blu-ray and high definition DVD (HD DVD), which encode video in high definition (HD) resolution have become increasingly popular.
These newer digital distribution formats include a wide array of added features designed to enhance the user experience especially when viewing video. These include newer and advanced video compression algorithms, surround sound audio, as well as many informative and interactive features delivered through multiple presentation layers in addition to a main video layer.
An HD DVD disc for instance, may include a background plane, a main video plane, secondary video plane, and subtitle graphics plane, among others. A Blu-ray disc may contain a presentation graphics plane and an interactive graphics plane, which are combined with a main video plane, to form final images for display. These additional layers are used to provide features such as improved subtitles, interactive menus, command buttons and the like.
The presence of additional layers or planes of video adds significantly to the complexity of tasks performed by a video player that processes them to form a final video image for display. The added tasks increase the computational load on the decoding and processing subsystems of a digital video player. Unfortunately this often requires an increase in the numbers of transistors included within the integrated circuitry of the video player and consequently leads to increased level of power consumption and heat dissipation.
In addition to decoding planes from several layers, the player must composite or combine corresponding decoded planes in each layer, to form a final image for display. This combining process often involves determining the transparency level or opaqueness of each pixel of each layer's plane and computing a final pixel using the corresponding pixels of each layer in relative proportion to their transparency. Methods for compositing digital images have been suggested, for example in T. Porter and T. Duff, “Compositing Digital Images”, Proceedings of SIGGRAPH '84, 18 (1984).
Straight-forward applications of known compositing methods can be very inefficient. As can be appreciated, compositing is a compute-intensive process, and can be potentially wasteful if performed on a large number of pixels that do not contribute to the final image. Accordingly there is a need to reduce the computational load and associated power consumption required to decode and efficiently display multilayered digital video.